Static pressure measurement is one of several tests routinely performed on airfoils in a wind tunnel. For a conventional open orifice, three effects combine to produce orifice induced pressure error. First, fluid flow turns into the exposed orifice. Second, fluid flow stagnates at the downstream edge of the orifice to produce a pitot effect. These two effects generally combine to provide a static pressure measurement which is too high; i.e., positive error. Third, orifice imperfections result in further positive error. Examples of orifice imperfections include an orifice which is out-of-round or has burrs, chamfering, a particle lodged inside, or a longitudinal axis which is not normal to the model surface.
To avoid the error resulting from orifice imperfections, conventional orifices must be carefully drilled normal to the airfoil surface. The orifices must be perfectly round, square edged, and clean.
Also, in order for the effects of orifice induced pressure error to be acceptably small at high Reynolds numbers and cryogenic temperatures, the orifices must have a small diameter (e.g., 0.010 inches). The fabricating of such small orifices with no imperfections requires much time, and therefore is very costly. In addition, the airfoil must be carefully handled after fabrication to avoid subsequent clogging of the orifices.
Accordingly, it is an object of this invention to reduce significantly or eliminate orifice induced pressure error during static pressure measurements of an airfoil.
It is a further object of this invention to reduce or eliminate orifice induced pressure error in an expeditious and inexpensive manner.
It is a further object of this invention to achieve the foregoing objects during high Reynolds number testing at cryogenic temperatures.
Other objects and advantages of this invention will become apparent hereinafter in the specification and drawings which follow.